Magnetic testing device



1937- J. PFAFFENBERGER 8,

MAGNETIC TESTING DEVICE Filed Aug. 27, 1934 '20 Pal. v F1552,

/5 1% A lb I4 1 24 l3 l2 I5 Fig.5. 2'

B2- I H 51- H lnverwtor:

Joachim Dfaffenbergar,

g H s Attorney I Patented New. 2, 337

UNEE

aerator MAGNHI'EEC rnsrnre nnvren Joachim Mafienberger,Berlin-Mariendorf, Germany, :ssignor to General Electric Company, acorporation of New York Application August 27, 1934, Serial No. 7411.702

lin Genny September 28, 1933 7 claims.

My invention relates to magnetic testing devices and methods andconcerns particularly arrangements for locating flaws in tested objectsand locating portions thereof lacking in homo-' 5 geneity.

It is an object of myinvention to provide a testing arrangement whereinthe tested object is subjected to a magnetizing field but the test issubstantially unaffected by variations in the m magnetic inductionresulting from variations in the strength of the magnetizing field orinevitable variations in reluctance of the magnetic circuit.

It is a further object of my invention to provide an arrangementavoiding false indications result- 15 ing solely from the nature of the.surface of the tested object which might readily occur, for example,when employing exploring elements with pole pieces to be brought incontact with the tested object having an uneven surface or a sur- 20face unevenly coated with scale or other nonmagnetic material.

It is still another object of my invention to provide an arrangementresponsive to deep internal flaws as well as to surface flaws.

:5 In carrying out my invention in its preferred form, I magnetize thematerial to be tested longitudinally in case of a tested object havingone dimension relatively small with respect to the others but employonly the transverse component of flux for making the measurement ortest. The test is made by observing the Barkhausen efiect' in the coreof an exploring coil passed along the surface of the tested object.

The features of my invention which I believe to be novel and patentablewill be pointed out in the claims appended hereto. A betterunderstanding of my invention, itself, however, may be obtained byreferring to the following description taken in connection with theaccompanying 9 drawing in which Fig. 1 represents schematically thearrangement of elements forming one embodiment of invention andillustrating a condition where the tested object is without flaws;

g. 2 illustrates the condition where there is a 5 faulty weld in thetested object; and Figs. 3 and a are magnetization curves representingthe relationship between field strength and fiuxdensity in the, core ofthe exploring element in thecases, respectively, where the tested objectis free from i flaws and where a flaw is detected by means of myapparatus.

Referring now more in detail to the drawing in which like referencecharacters are used to designate like parts throughout,-I haveillustrated in Figs. 1 and 2 an object it to be tested which is in theform of two steel plates l2 and 63 joined by a butt weld iii. Althoughmy invention is particularly advantageous for testing welds, it'

will be understood, of course, that the invention is not ted thereto andis useful for detecting flaws such as breaks, cracks, fissures, slag,and other points lacking in homogeneity in any part of a specimen ofmagnetic material. The specimen or tested object it is magnetizedlongitudinally or along one of the greater dimensions 5 in the case ofsheets, in any suitable manner as by means of magnetizing windingssurrounding the object or by means of magnets 05 and it brought incontact with two points on the object on either side of the portion tobe tested for flaws. For testing welds the direction of magnetization ispreferably transverse to the line of the weld. The magnets l5 and itmaybe either electro-magnets or permanent magnets. If electro-magnetsare employed, they may be energized by either alternating or directcurrents.

The detecting element ll consists of a core of magnetic material andmeans for observing the Barkhausen efiect therein. An arrangement forobserving the Barkhausen efiect which I have found satisfactory consistsof a winding E8 on a core ll connected to a pair of headphones or othersound-producing device through an amplifier 20. The detecting element iiis adapted to be moved along the tested object H with the magnetic axisof the detecting element ll perpendicular to the direction ofmagnetization of the tested object H.

By making the detecting element ll responsive only to the normalcomponent of the field or the stray field around the tested object ll,theeifects of variations in the nature of the surface of the testedobject, and variations in the reluctance across the contacting surfacesof the magnetizing magnets to and i6 and object H, as well as variousother sources of error are eliminated.

Although, if desired, means may be provided for passing the detectingelement H mechanically along the tested object H or for producing themotion of the detecting element A? automatically, I find thatsatisfactory results may be obtained by merely passing the detectingelement ill along the tested object it manually since no particular typeof motion is necessary.

The curve 2i in Fig. 1 represents the strength of the transversecomponent of the magnetic field acting on the object it from one magnetto the other. It will be observed that this field varies from a, maximumof one polarity at one end to a maximum of the opposite polarity at theother and and, in the case of a homogeneous uniform tested object, thevariation in field strength is uniform and passes through zero at thepoint midway between the magnets to and it. In the case illustrated,this happens to be the point at which the weld it is located as the weldit; represents a point which is homogeneous and without flaws.

As is well known, when a magnetic material is subjected to a varyingmagnetizing field, the flux density does not vary continuously butvaries uniformly in steps or jumps, producing what is known as theBarkhausen effect, described by Professor Barkhausen in 1919 in thePhysik Zeitschr, volume 20; pages 401 to 403, and by Richard M. Bozorthin the Physical Review, volume 34, pages 7'72 to 784, September 1,v1929, and volume 39, page 353, January 15, 1932. Consequently, if apair of head-phones i9 is connected to the wind ing l8 carried by thedetecting element 11, a note will be heard in the head-phones as thedetecting element 11 is passed along the object as a result of theuneven variations in magnetization of the element ll inducing voltagepulsations in the winding I8. As long as the transverse component offield strength represented by the curve 2| varies smoothly, the noteproduced will be of a low intensity and will not vary perceptibly instrength as the detecting element IT passes from a point 22 on onesideof the weld M to a point 23 on the other side of the weld [4. However,if there should be a flaw in some portion of the tested object II as onecaused by imperfect weld 25, (Fig. 2) for example, there will be a peak25 in the portion of the field strength curve 26 opposite the imperfectweld 24, owing to the fact that the magnetic flux lines are deflected atthis point by the high reluctance at the defect. In passing detectingelement ll across the point at which the peak; 25 .in the magnetizationoccurred, the detecting element l'l will be subjected to a relativelygreat change in magnetization, producing a much louder note in the headphones l9 and causing the flaw at 24 to be detected.

Referring to Fig. 3 which represents the variation in flux densityplotted along the B-axis with variations in field strength plotted alongthe H- axis it will be seen that, as the field acting on the element llchanges by a small constant value corresponding to the movement ofdetecting element I! from point 22 to point 23 (Fig. l) the fiux densityin the detecting element l1 changes a small amount from B1 to B2. Thechange in flux density being uneven, the Barkhausen effect, a note isproduced but only one of low intensity. However, if there should be aflaw producing a peak in the field strength curve, there will be aconsequent pronounced increase in flux density from B1 to B: asillustrated in Fig. 4, and a pronounced increase in the intensity of thenote heard in the head-phones l9 will be produced.

Preferably the iron core of the detecting element I1 is relativelyshortand compact giving it a relatively great demagnetization factor andmaking the Barkhausen effect substantially independent within widelimits of the field strength and dependent only upon the amount ofchange in the field. The Barkhausen effect may, it will be understood,be increased by employing a material having a high hysteresis with ahigh initial permeability for the magnet core of the detectingrelementl1.

Since the element l1 need not be moved in a particular manner and sinceits action depends merely on the change in field strength as it is movedalong, it may, if desired, be made very small and covered with aprotective insulating shield permitting it to be passed along portionsof a tested object difllcult of access, such as the interior of pipesand similar objects for the purpose of testingseams and the like.

In testing relatively thin plates where the magnetic skin efiect is ofless consequence, altemating-current electromagnets may advantageouslybe employed at l5 and IQ for magnetizing the object H. Since thefrequency of the commercial alternating systems is usually 60 cycles orless, the frequency of the voltage impulses produced by the alternationsin polarity of the power source will be considerably less than the faultindications given by Barkhausen effect as the frequency for this purposeis considerably higher. Both the difference in frequency and thesensitivity to higher frequencies of the ordinary amplifier andheadphones as well as the human ear permit readily differentiating thenote produced by the Barkhausen effect from the low frequency humproable material which comprises magnetizing the object, passing adetecting coil with a magnetic core along the object in a directionparallel with the direction of magnetization with the magnetic axis ofsaid core perpendicular to the direction of magnetization of saidobject, thereby producing voltage pulsations in said coil due toBarkhausen eifect in said core, and observing the points at whichpulsations of increased strength occur in the voltages induced in saidcoil.

2. A method of testing an object composed of magnetizable material whichcomprises magnetizing the object, passing a detecting element includinga magnetic substance along the object to be tested, and observing theBarkhausen effect in the detecting element.

3. A method of testing a weld in magnetizable material which comprisesmagnetizing the material longitudinally but transversely to the weld,passing a magnetic element along the material transversely to the weld,and observing the Barkhausen effect in the magnetic element.

4. A method of testing an object of magnetizable material whichcomprises magnetizing the object in a given direction, passing adetecting coil with a magnetic core along the object in said direction,thereby producing voltage pulsations in said coil due to Barkhauseneffect in said core, and determining the points at which variationsoccur in the pulsations in the voltages induced in said coil.

5. A device for testing an object composed of magnetizable materialwhich comprises an element composed of a magnetic substance and adaptedto be passed along said object, and means for observing the Barkhauseneffect in said element.

6. A device for testing an object composed of a magnetizable materialwhich comprises a detecting coil with a magnetic core composed of asubstance having a high hysteresis with a low initial permeabilityadapted to be passed along said material, and means responsive to thevoltages induced in said winding.

'7. An arrangement for testing an object com.- posed of a magnetizablematerial which comprises means for magnetizing the object. an elementcomposed of a magnetic substance adapted to be passed along said object,and means for observing the Barkhausen effect in said element.

JOACHIM PFAFFEN'BERGER.

